On unexpected behavior of viscosity of diethylene glycol-based MgAl2O4 nanofluids

The article is a continuation of research on the rheological properties of diethylene glycol-based MgAl2O4 nanofluids. Previous article [Zyla et al., RSC Adv., 2013, 3, 6429-6434] presents the results of research on the rheological properties of the material. The results have shown an unexpected behavior of the viscosity curves for this material provided that the dynamic viscosity of the nanofluid in a certain area of the shear rate decreases, then increases, and beyond a certain shear rate declines. Additionally, it has shown that this effect can be observed only with the use of freshly prepared samples. If the samples are subjected to long-term shear, the experiment results with pseudoplastic flow, with greater dynamic viscosity than in the case of fresh samples. This paper is an attempt to explain this phenomenon. The authors have conducted a series of experiments using RheoScope in the range of shear rates from 1 s(-1) to 1000 s(-1) and Rheo-NMR. Studies have shown that during the time of rotational measurements nanoparticles agglomerate in suspension, which causes the unexpected shape of the viscosity curves of the material.